The amount of worldbuilding seen in thee show is impressive; there's a detailed system of magic, several historical events are mentioned, and in one episode the wheres and whys of how girl trolls get their magic is explained. They even have a different calendar system.
In "Bringing Back the Magic," the girls had to face a cyclops without their magic, and with the Ancients' lives at stake. Though it takes a lot of effort, they manage to defeat it.
The arc where the girls turn evil, letting Simon take over Trollzopolis. He pulled off quite an evil plan.
Rock defeating Simon by smashing his trophy against his head.
The girls stopping the Trollhiti volcano from erupting in the last episode.